A multi-leaf collimator of a radiotherapy apparatus includes groups of leaves, a main material of which is heavy metal, such as tungsten, and the leaves in each group are closely adjacent. Pairs of the groups of leaves are positioned in a radiation direction of a radiation ray. The groups of each pair move in close and opposite directions, mutually. A drive unit which moves each leaf in a conventional radiotherapy apparatus includes drive gears which contacts cogs formed in edges of leaves and are connected to a motor via shafts. The drive unit is described in Japanese Patent Disclosure (Kokai) No. 2002-253686, page 3 and FIG. 12, for example. Since it is required to move each leaf in the close and opposite directions according to a range of the radiation ray, namely a diseased part to be treated, a driving mechanism is provided with respect to each leaf
A conventional multi-leaf collimator includes the groups, each of which has about 40 adjacent leaves with a thickness of about 3 mm. Although it is theoretically possible to approximate the radiation range to the medical treatment range by reducing thickness of the leaves and increasing number of leaves, it is actually difficult to reduce the thickness of the leaves due to the drive gears which move the leaves and the shafts which connects the drive gears and the motor. Moreover, it is also a problem that the drive unit increases in size and in weight.
Furthermore, backlash could occur in such a gear mechanism, and accuracy of move control of the leaf is reduced. Therefore, when the radiation range is set, positions of the leaves are shifted, it is difficult to accurately set the radiation range, and it could be a problem that the radiation ray is radiated to a normal part of a patient. The above mentioned Japanese Patent Disclosure (Kokai) No. 2002-253686 discloses a gear mechanism which avoids the backlash, however the gear mechanism increases in size.